Friday, June 1, 2007
Safety Update: Critical Links
Any effort to boost rotorcraft safety will have to include a close look at the role helicopter mechanics play.
OF THE VARIETY OF CAUSES OF HELICOPTER accidents, perhaps one of the most troubling is that of maintenance error. Two recent accidents illustrate the vulnerability of rotorcraft to lapses in diligence by mechanics. A third in the not so distant past may be a frightening case of what can result from lapses in character.
The most recent case is that of a Robinson R44 Raven 2 operated by Silver State Helicopters that crashed on a training flight on March 27 in Ponte Vedra Beach, Fla. The crash killed the instructor pilot and her student.
The Raven 2 was on its first flight after a 30-min maintenance test flight by the mechanic and a Silver State pilot, according to a preliminary NTSB report. It had undergone a 100/300-hr inspection and the removal and replacement of mast fairing ribs. The latter required removal of the main-rotor swashplate push/pull tubes.
Silver State personnel told the NTSB the flight had been planned to use an R22 between 0900 and 1100 local time, but a scheduling conflict left none available. Management then allowed the instructor and student to conduct the orientation and familiarization flight in the R44. They took off from Jacksonville, Fla.’s Craig Municipal Airport (CRG) at about 1010 for a flight south along the Atlantic coast to St. Augustine and back to CRG.
Several witnesses observed the helicopter cruising south along the coast at 200-500 ft agl. One, a former pilot and U.S. Army helicopter crew chief, said he saw the R44 in straight and level flight, then heard a change in "rotor noise, followed by a bang/pop/twang sound," according to the safety board. He said the helicopter then "snap-rolled" left and crashed in a nose-low attitude at about 1030. The helicopter hit the sand, bounced about 100 ft, then came to rest near the low-tide water line. It began to burn, but the fire extinguished itself quickly.
The main wreckage consisted of the fuselage, main-rotor assembly, tail boom and tail rotor. Pieces of the fuselage and skid tubes were scattered between the initial impact and main body of wreckage. The Lycoming IO-540 engine separated and came to rest next to the main wreckage.
With the tide coming in, an FAA inspector had the wreckage moved to CRG, where it was examined by the NTSB investigator-in-charge, FAA inspectors, and representatives from Robinson and Lycoming. Besides the fire and impact damage, they noted some interesting things.
The right forward servo-to-swashplate push/pull tube fitting was disconnected and its attach hardware — a bolt, lock nut, two washers, and pal nut — was missing. That fitting in the left forward position was connected, but the lock nut was found partially engaged on the bolt threads. Its torque was "finger tight," the NTSB said, and no pal nut was noted. The aft servo and push/pull tube fitting was secured with the appropriate hardware.
On the R44, the swashplate push/pull tubes are connected to the swashplate assembly and the upper linkage of the servos. The servos’ bottom linkages are connected to bell crank/fork assemblies, which connect to the cyclic and collective through a series of push/pull tubes and bell cranks. The three servos support the main-rotor swashplate, the NTSB said, and hydraulically boost the main-rotor flight controls to eliminate cyclic and collective feedback forces. The system would not have been able to function properly without complete linkages. It is not considered likely that the right forward push/pull tube fitting hardware was missing when the R44 took off.
The Silver State mechanic was described by one person familiar with the investigation as a sharp guy who had been to the Robinson school. He reportedly had been called away from another task when he was re-assembling the main-rotor swashplate push/pull tubes. Then his shift ended. When he returned for the next shift, he conducted the maintenance test flight with the company pilot, then signed the aircraft’s return to service.
A Bell Helicopter mechanic involved in the other recent case bears less of a burden. His lapse only cost a multi-million prototype of the U.S. Army’s new Armed Reconnaissance Helicopter. When he assembled a fuel boost pump for the No. 4 prototype, he attached the check valves without removing the plastic shipping caps that covered the ports for those valves. The valves cut through the plastic, leaving one top of a cap in each of the redundant fuel lines for the Honeywell HTS900 engine. About 20 min into the prototype’s first flight, the engine flamed out. The pilot executed a near-perfect autorotation, but his skids dug into soft ground. The ARH tipped, rolled, and was destroyed.
The last case reminds us of the terrifying truth that there are no safeguards against errors that are intentional. To be fair, the account relies on court proceedings, and those involved may yet be found innocent. But a New Zealand court found grounds to try a maintenance shop owner and a supervising mechanic for manslaughter in an August 2005 crash that killed a Robinson R22 owner and his passenger. The owner had picked up his R22 after replacement of the tail-rotor drive shaft, rotor blades and aft flexplate coupling. He crashed on his second flight that day. Investigators found the flexplate coupling had been assembled incorrectly. The shop owner and mechanic are accused of failing to supervise an unlicensed mechanic who struggled to assemble it.